The present invention relates to a method for producing an R-Fe-B-based, sintered magnet wherein R is one or more of rare earth elements including Y.
An R-Fe-B-based, sintered magnet may typically be produced by melting and casting alloy metals for the magnet to form an alloy ingot, pulverizing the ingot to alloy powder, molding and sintering the alloy powder, heat-treating the sintered body and then working it. For the purpose of finely pulverizing coarse alloy powder, a jet milling method comprising impinging coarse alloy powder each other in a high-pressure inert gas atmosphere may be utilized to produce dry, fine powder, and a wet powder method such as a ball milling method, a vibration milling method, etc., in which a magnet alloy is pulverized in an organic solvent, may also be utilized to form a mixture of fine alloy powder and an organic solvent which is then dried to provide dry alloy powder. The formation of a green body from the dry alloy powder may be conducted by introducing a measured amount of the dried alloy powder into a die cavity, and then compressing the alloy powder in the die cavity while applying an orientated magnetic field. Alternatively, the dry alloy powder may be introduced into a die cavity to which a magnetic field is applied in advance and then compressed.
Since pulverized R-Fe-B-based magnet is chemically extremely active, it is highly likely that the R-Fe-B-based magnet powder is rapidly oxidized in the air, resulting in the deterioration of magnetic properties. Japanese Patent Laid-Open No. 61-114505 and U.S. Pat. No. 4,911,882 disclose a method for preventing the oxidation of R-Fe-B-based magnet powder, in which starting material powder is mixed with an organic solvent, the resulting mixture is introduced into a die cavity in the same manner as for the dry alloy powder and compressed in a magnetic field, and the resulting green body is dried, sintered and heat-treated. By this method, the deterioration of magnetic properties by oxidation can be prevented because the alloy powder is compressed into a green body in an organic solvent. However, regardless of whether the dry alloy powder or a mixture of alloy powder and an organic solvent is used, the above method fails to provide a sintered magnet with excellent magnetic properties which are potentially owned by the R-Fe-B-based magnet powder, meaning that the resultant R-Fe-B-based, sintered magnet shows unsatisfactory magnetic properties.
As a result of analysis as to why the above conventional method fails to draw satisfactory magnetic properties from the R-Fe-B-based magnet powder, the inventors have found the following facts:
(a) Pulverized R-Fe-B-based magnet particles show a strong interaction between themselves because they have several times as large a coercive force as that of a ferrite magnet, so that fine powder is likely to form bridges between them. Accordingly, even in a magnetic field the R-Fe-B-based magnet powder is not always well orientated locally in a die cavity. PA0 (b) The fine R-Fe-B-based magnet powder orientated to a certain level in a die cavity by a magnetic field applied thereto is likely to be disturbed with respect to its orientation by a compression force in the process of forming a green body. The extent to which the orientation of the fine R-Fe-B-based magnet powder is disturbed is large when the direction of a magnetic field applied is in parallel with the direction of compression, namely in the case of a vertical magnetic field. However, the disturbance of the orientation of the fine R-Fe-B-based magnet powder may take place also when the direction of a magnetic field applied is in perpendicular to the direction of compression, namely in the case of a transverse magnetic field.
Because of the above two phenomena, the fine R-Fe-B-based magnet powder is not well orientated in a die cavity by the above conventional method, resulting in a sintered magnet with poor residual magnetic flux density and maximum energy product.